A typical optical waveguide (WG) has a core layer having a high refractive index, surrounded by cladding layers of lower refractive index. The refractive index boundary acts to guide photons reaching the boundary between core and cladding back into the core.
Electromagnetic waves propagating in optical waveguides create evanescent electric and magnetic fields in lower refractive index cladding regions adjacent to the higher refractive index waveguide core. A propagation constant along the axis of the waveguide core and field distributions transverse to the waveguide core depend on the value of the refractive index in the cladding regions. Refractive index sensors have a waveguide core in close proximity to a sample region that essentially forms a part of cladding. Changes in the refractive index within the sample region that overlap field distributions alter the electromagnetic mode profiles and propagation constants of guided waves. Changes in concentration of analyte molecules close enough to the core can change the average refractive index near the core if they have a different refractive index from the host media or solvent that they displace in or near the cladding. The altered refractive index may change propagation in the core, or alter amount of light escaping from the waveguide. Optical waveguides with particular coatings can sense the presence or concentration of certain molecules near the waveguide core.
Previously described are optical waveguides for sensing refractive index and conditions that alter refractive index within the evanescent fields near the waveguide core. Evanescent field optical waveguide sensor devices include Mach-Zehnder interferometers and ring resonators. Similar principles apply to waveguide grating devices and surface plasmon sensors which also provide a sensing mechanism within evanescent fields. Changes in the guided electromagnetic wave's propagation constant may be sensed via changes in the phase of the wave at some point where the light is mixed with a reference beam, as with an interferometer. Alternatively, a change in the propagation constant may alter the resonant wavelength of an optical waveguide resonator. Changes in the propagation constant may also be manifested in the angle of diffraction from a waveguide grating for a fixed wavelength or the angle of light coupled to a surface plasmon.
Recently, the local evanescent array coupled (LEAC) sensor has been developed. LEAC sensors make use of altered refractive index to change the amount of light escaping from the waveguide into a photodetector. LEAC sensors sense changes of refractive index in an upper cladding, or in a fluid that acts as an upper cladding, by altering evanescent coupling of a guided optical wave in a thin core 102 (FIG. 1), often 200 nanometers or less thick, through thin lower cladding to a nearby photodetector 106, 108. The altered coupling results in changes in an effective leakage of photons from the waveguide into the photodetector. The waveguide may have lateral cladding 104 or other provisions to prevent lateral escape of light from the core. U.S. Pat. No. 8,349,605 ('605) teaches a LEAC sensor which employs either a single photodetector, or multiple photodetector segments 106, 108 separated by insulating regions 110, and as shown in '605 FIG. 8B. Such insulating regions have been implemented by etching a thin layer of photodetector material into isolated photodetector elements, and filling the intervening regions with an insulator.
The evanescent or evanescently coupled optical field of the LEAC sensor must have at least some overlap with the photodetectors in order to generate photocurrent. However, as the optical field impinges on photodetector 106, 108 and intervening insulator 110 material of different refractive index, some amount of light is reflected or scattered from the discontinuity in refractive index in a direction parallel to the waveguide core's 102 axis. The scattering of light is disadvantageous because it reduces the optical power remaining in the guided mode, reducing the magnitude of photocurrent generated by subsequent photodetectors. The scattering of light is also disadvantageous because it increases the background photocurrent in neighboring detectors by a mechanism other than the desired mechanism of evanescent coupling.
Fabrication of segmented photodetectors in a LEAC sensor can further lead to non-planar waveguides unless complex, and hence costly, fabrication is employed. When the insulation is deposited, typically with chemical vapor deposition or plasma enhanced chemical vapor deposition, to fill the etched regions between isolated photodetector elements, the insulating material is also deposited on top of the photodetector elements leading to a non-planar surface. In the prior art, chemical-mechanical polishing was used to make the surface planar prior to depositing more layers, such as lower cladding (not shown in FIG. 1) or core of the waveguide. Precision chemical-mechanical polishing requires specialized equipment and processes, and increases the complexity and cost of LEAC sensor fabrication.